The rodent trigeminal system has favorable characteristics for elucidating mechanisms of development of brain pathways and order in them. Each whisker on the face projects to a discrete cell group in the brain. In the cerebral cortex these are called barrels. The ensemble of barrels makes a map of the animal that can be recognized in tissue sections with or without stains. We propose to exploit the map that the barrels provide, the postnatal development of the intracortical connections within the map and the sensitivity of that development to changes in sensory input to clarify how specific intracortical circuits required for integration of inputs from the whiskers develop. We will examine the following hypotheses: . Hypothesis I: there is a sequence of development of intracortical (interbarrel) projections that is lamina specific, . Hypothesis II: connection to the ordered periphery is necessary for the proper patterns to develop, . Hypothesis III: the effect of selected whisker removal is asymmetrical at the boundaries between connected barrels and disconnected barrels which are at a competitive disadvantage, . Hypothesis IV: normal neuronal activity plays a role in the development of interbarrel connections, . Hypothesis V: intracortical action potentials play the major role in the development of the interbarrel connections. We will use lipophilic dyes, targeted under direct vision to individual barrels or to particular cortical layers over and under individual layer IV barrels and other transported markers, to demonstrate intracortical connectivity. The project will use CORE C to digitize and visualize results of fluorescent dye transport, CORE B to quantitate these results and the results from transported marker studies and CORE E to align findings from serial sections. These studies relate directly to work in Projects 4, 6, and 7. The goal is to understand the manner in which intracortical communication. The work can contribute to clarifying mechanisms that may be important for understanding mental retardation, epilepsy and learning disabilities.